1. Field of the Invention
The present invention relates to a furnace having sequentially arranged furnace sections including strategically located electric induction heating sections to heat ferrous workpieces to a predetermined furnace discharge temperature without workpiece losses due to changes to required heating rates arising out of changing dimensions and metallurgical composition of the workpieces, and, more particularly, to a method and apparatus for initial heating of ferrous metal workpieces by a rapidly responsive induction heating furnace section to supply heated workpieces at selected temperatures for heating in a constant temperature chamber of a sequential furnace section to produce a predetermined workpiece discharge temperature and for additional heating of the workpieces by a further rapidly responsive induction heating furnace section to a controllable discharge temperature without adverse limitations due to changing magnetic properties at Curie temperatures of the workpieces and for further heating in a constant temperature chamber by a fourth sequentially arranged furnace section for attaining a final furnace discharge temperature suitable for cleaning, annealing or other purposes requiring heating of the workpieces and/or the supply of heated workpieces for further processing particulary galvanizing.
2. Description of Related Art
Furnaces comprising multiple furnace sections are known in the art for heating elongated ferrous metal workpieces having any of diverse cross-sectional shapes and passed in a generally continuous fashion in the furnace in an end-to-end relationship. Examples of such ferrous metal workpieces are wire, bar stock, structural shapes, plates, rails and strip. While not so limited, the present invention is particularly useful for the heating of ferrous metal workpieces known in the steel making industry as steel strip. Steel strip in a coil has a substantially uniform end to end thickness and when supplied from a hot strip rolling mill installation is in a coiled form having a strip thickness in a thickness range of, for example, 0.50 to 0.025 inch, and when supplied from a cold rolling mill installation is in coiled form having a strip thickness in a thickness range of, for example, 0.07 to 0.006 inch. These ranges of strip thicknesses are only generalizations and should be expected to vary considerably when comparing specific steel making facilities.
A significant portion of strip production is lost because of the inability to maintain the quality of the heat treating cycles during a transition from one strip gauge or thermal cycle to another strip gauge or thermal cycle. In a steel making facility, it is important to utilize heat treating equipment embodying a design and capacity to rapidly adjust the heat treating parameters to meet the need for changing from one strip gauge or thermal cycle to another without significant losses due to a non prime strip product. Such losses are of less concern in a continuous annealing process, for example, where extensive production with strip having the same or similar metallurgical properties dominate the product mix. However, the more frequent and drastic changes to the strip thickness and metallurgical properties as needed, for example, to fulfill the sales of small strip tonnage usually required the use of batch annealing operations to remain cost effective however batch annealing operations are time consuming when compared with the speed at which annealing can be accomplished in a continuous annealing line. This is especially important when the annealing operation is part of the hot dipped galvanizing process used to adhere a protective coating of zinc and zinc compounds on the surface of the strip product. The heat treating process for the strip generally requires heating the strip to a temperature greater than the strip entry temperature into a bath of molten zinc for the galvanizing process. The heat treatment is usually necessary not only to clean the strip of mill scale, oil and other surface contaminants, but also to heat the strip to a much higher temperature for annealing because the strip supplied from a hot or cold rolling mill operation in a metallurgically hard condition which is usually not a desired property of galvanized strip.
When the metal workpiece is a carbon steel strip and the annealing furnace is used in a galvanizing process line, three hardness grades of the strip products are generally produced namely: full hard; commercial quality; and drawing quality. For a metallurgically full hard strip product, the hardness of the strip as received from the hot strip mill or the cold rolling mill is not significantly changed because of the low temperate excursion in the heat treating process. The nominal furnace exit temperature is in the range of 1000.degree. F. to 1020.degree. F. For a strip hardness of commercial quality, the strip is heated to a nominal furnace exit temperature in the range of between 1345.degree. F. and 1400.degree. F. For strip hardness of drawing quality, the strip is heated to a nominal furnace exit temperature in the range of between 1540.degree. F. and 1600.degree. F.
A continuous annealing furnace is typically made up of three furnace sections operating within limited variations to the furnace heating capacity when the steel product to be annealed does not significantly change. If a significant change to the product did occur then either a line speed change or a change to the average specific heating values by either the cleaning or soaking furnace sections will be needed due to interrelated optimum process parameters which were part of the design an operation of these furnace sections. In a heat treatment process for a commercial strip, the optimum cleaning requires heating the strip to a temperature of 1150.degree. F. which can be achieved as a strip exit temperature from a first furnace section. Since the Curie temperature for carbon steel is about 1325.degree. F., depending on constitute alloys the second furnace section must heat the metal strip to a temperature above the Curie temperature for the strip to produce a drawing quality product. When the second furnace section has the form of an electric induction furnace, longitudinal flux inductors can be used to achieve high efficiency heating of the carbon steel strip to a temperature below the Curie temperature. The third furnace section is typically used for heat soaking the strip with a relatively small temperature increase.
An example of a multiple furnace section known in the art is found in Japanese Patent Publication 57-19336 laid open Feb. 1, 1982, entitled Continuous Annealing Furnace Having Induction Heating Section and provides an arrangement of furnace sections made up of an induction heating zone between an upstream gas heating zone and a downstream soaking zone. Downstream of this furnace arrangement there is an induction reheat zone between an upstream gas reheat zone and a downstream slow cooling zone. This annealing furnace arrangement suffers from the disadvantage that the strip is initially heated in each arrangement of furnace sections by gas heating zones which cannot be controlled to respond to different required heating rates for strip that changes from coil-to-coil. Japanese Patent Publication 55-170276 laid open Dec. 4, 1980 entitled Continuous Annealing Method discloses the temporary use of induction heaters having high heating rates in the heating zone and/or soaking zone for changing heating cycles and after the heating cycle has been adjusted, the strip speed is altered to eliminate the need to use the induction heaters.
U.S. Pat. No. 4,239,483 discloses a continuous annealing facility with a preheat zone divided into units each having nozzles that can be turned ON and OFF to selectively direct high speed heating gas against the strip. During a transition between strips of different thicknesses, the temperature in the preheat zone is controlled by the number of gas injecting preheating units in operation. Downstream of the preheat zone is a rapid heating zone. The temperature in the rapid heating zone changes to a preset value based upon the thickness of the strip in the line.
It is an object of the present invention to provide a furnace and a method for heating ferrous workpieces to minimize off prime heat treatment by a unique placement of at least two induction heating furnaces each followed by an additional furnace section to alter heating of workpieces rapidly that enter the furnace in a continuous end to end relationship for heating each workpiece according to predetermined heating requirements.
It is another object of the present invention to provide an electric induction heating furnace section having at least one induction coil pair supplied with electrical power for heating elongated ferrous workpieces selectively by a transverse magnetic flux field or a longitudinal magnetic flux field depending on the Curie temperature by a furnace having a plurality of additional furnace sections.
It is a further object of the present invention to provide a plurality of pair of sequential furnace sections and a method for heating ferrous workpieces fed in an end-to-end relationship with the first furnace section of each pair being rapidly adjustably controlled to heat a workpiece in response to a desired delivery temperature for the workpiece delivered from the second of that pair of furnace sections.
It is still a further object of the present invention to provide a heat furnace for workpiece in a galvanizing line in which the workpieces are selectively heated to a temperature above or below the Curie temperature by selecting a phase relation to current applied to an electrical inductor coil pair.
It is another object of the present invention to provide an annealing furnace with multiple electric induction heating sections that will continuously heat any combination of varying product grades with minimum transition material loss between changes in grades.